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UCC256402: Open-loop working mode set-up

Part Number: UCC256402
Other Parts Discussed in Thread: UCC25600

Hi experts,

My project needs the LLC converter to work as a DC transformer and the switching frequency constantly working at the resonant frequency.

 In my project, the LLC converter resonant frequency is set as 100kHz, so my question is how should set up the switching frequency and fix it at 100kHz?
Because I didn't see any description about the design of the norm switching frequency in the datasheet.
Furthermore, there is not any description of the open-loop working mode in the datasheet also, so i am wondering what if i connect the FB to ground, will it work in open-loop mode?


  • Hi Hong,

    Running UCC256402 open loop would require some modifications to the VCR circuitry as well as the FB pin to sink a constant current out of the FB pin. Connecting the FB pin to ground would make UCC256402 to not switch. Since the FB pin is loosely regulated to ~5.6V, you could set the switching thresholds for VCR by connecting a resistor from FB to ground that sinks a fixed current (somewhere between 0uA and 82uA). The amount of current would determine the switching thresholds for VCR. You could depopulate the top VCR capacitance and then the frequency is only dependent on the internal 2mA ramp current and whatever capacitor you select for VCR. 

    UCC25600 would be a simpler way of achieving an open loop LLC as this controller would just require setting a resistor between the RT pin and ground to fix the switching frequency (use equation 18 in the datasheet). 

    Best Regards,

    Ben Lough

  • Thanks, it helps me a lot.  But may i ask another question?

    Is there any similar LLC controller IC with UCC25600 but with integrated Driver ability?


  • Hi Hong,

    TI does not have a controller similar to UCC25600 with the driver integrated unfortunately.

    Best Regards,

    Ben Lough